Genetics Part II: Probability and Pedigree

Nestor T. Hilvano, M.D., M.P.H.(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain,

Textbook, 2012.)

Learning Objectives

1. Explain the rule of multiplication and the rule of addition to determine the probability of an event in monohybrid crosses.

2. Explain how family pedigrees can help determine the inheritance of human traits.

3. Discuss and provide examples of how recessive and dominant disorders are inherited.

4. Describe the inheritance patterns of pleiotropy and polygenic inheritance.

Laws of Probability Govern Mendelian Inheritance

• The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.

# of actual occurrences Probability = ______________________

# of possible occurrence

• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

• Ex: flipping heads on a coin= ½ (50%)

Rules of Probability• Independent events – ex.

Tossing of coin • Rule of Multiplication –

probability of 2 events occuring together is the product of the probabilities of the 2 events occuring apart

• Ex. Probability of recessive phenotype occuring in monohybrid? ½ x ½ = ¼

dihybrid? ¼ x ¼ = 1/16

• Rule of Addition – probabilities are added if there is more than one way an outcome can occur, as in determining the chances for F2 heterozygous offspring

• Ex. Probability of Bb is

¼ + ¼ = ½

Degrees of Dominance • Complete dominance occurs when

phenotypes of the heterozygote and dominant homozygote are identical

• Incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

• Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways; Type AB blood is an example

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Incomplete dominance In snapdragon

What is the Phenotype ratio? _____________

Figure 9.8A

Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Pedigree Analysis

• Pedigree is a family tree that describes the interrelationships of parents and children

• Inheritance patterns of particular traits can be traced and described

• can be used to make predictions of future offspring

• Dominant-recessive inheritance• Apply Mendelian laws

Brachydactyly: dominant condition marked by short/clubbed fingers & toes

Recessive Disorders• Most common; affected children are

homozygous recessive • Born to normal parents who are both

heterozygotes (Dd) • Ex. Deafness, albinism, cystic fibrosis, PKU,

sickle-cell disease, etc. • Practice: construct punnett squares• Parents – both Dd (normal but are carriers)• Gametes: D and d• Probability for Hearing (DD, Dd)= ¾ (75%); Deaf (dd)= ¼

(25%); Carriers (Dd)= 2/4 (50%)

Dominant Disorders

• Homozygous dominant causes death of embryo; only heterozygous have this disorder ; 50% chance of passing the condition

• Ex. Achondroplasia (dwarf), extrafingers or web digits, Huntington’s disease, and Hypercholesterolemia

Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

Achondroplasia: a dominant trait disorder

Pleiotrophy and Polygenic Inheritance

• Pleiotrophy = single gene produce multiple phenotypic characteristics. ex: sickle cell disease (ss) – confers phenotypic traits as anemia, weakness, mental dysfunction, organ damage, etc.

• Polygenic = 2 or more genes produce a single phenotypic characteristic. ex: mixtures of 3 genes (each w/ 2 alleles) for phenotype of skin color

Crossing Over • can separate linked alleles

• Producing gametes with recombinant chromosomes (crossing over of linked genes)

A B

a b

Tetrad Crossing over

A B

A b

a b

a B

Gametes

Sex-linked Disorders in Humans• Sex-linked genes determine the sex of individual and

other inheritable characteristics • Sex-linked disorders are due to recessive alleles• For a recessive sex-linked trait to be expressed

– A female needs two copies of the allele– A male needs only one copy of the allele

• Sex-linked recessive disorders are much more common in males than in females

• ex. Hemophilia (lack protein for blood clotting),

Color blindness (red-green), Duchenne

muscular dystrophy

Homework

1. Define terms: recessive disorders; dominant disorders; polygenic inheritance; pleiotrophy; pedigree; recombinant chromosomes, and sex-linked genes.

2. Describe the rule of multiplication and rule of addition in determining the probability of inheritance.

3. Male with black hair (Bb) married a female with red hair (bb). Construct a punnett square to show the location of gametes and offspring. What is the probability of having a black hair? Probability of red hair?

4. Whys is color blindness is more common in males?